Direct Control Signaling in a Wireless Communication System

ABSTRACT

A first wireless communication device belongs to a first of multiple groups of wireless communication devices. Devices in any given group are synchronized to the same timing reference and devices in different groups are not synchronized to the same timing reference. The first device receives a message that indicates, for each of one or more of the groups, a range of possible values for misalignment between the timing reference of that group and a common timing reference. The range accounts for uncertainty in that misalignment. The first device determines, based on the one or more ranges, intervals of times during which direct control signaling is expected to be received at the first device from one or more devices in one or more other groups. The first device then adjusts intervals of times during which it is configured to operate in an awake state to narrowly encompass the intervals of times during which the direct control signaling is expected to be received.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/413,863, which was filed on Jan. 24, 2017, which is a continuation ofU.S. application Ser. No. 14/405,051, which was filed on Dec. 2, 2014,which is the national stage application of PCT/SE2014/050923, filed onAug. 8, 2014, and claims benefit of U.S. Provisional Application61/864,397, filed Aug. 9, 2013, the disclosures of each of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present application relates generally to method and apparatus in awireless communication system, and specifically to method and apparatusfor direct control signaling in such a system.

BACKGROUND

Device-to-device (D2D) communication is a well-known and widely usedcomponent of many existing wireless technologies, including ad hoc andcellular networks. Examples of device-to device communication includeBluetooth and several variants of the IEEE 802.11 standards suite suchas WiFi Direct. These systems operate in unlicensed spectrum.

Device-to-device communications as an underlay to cellular networks havebeen proposed as a means to take advantage of the proximity ofcommunicating devices and at the same time to allow devices to operatein a controlled interference environment. Typically, it is suggestedthat such device-to-device communication shares the same spectrum as thecellular system, for example by reserving some of the cellular uplinkresources for device-to-device purposes. Allocating dedicated spectrumfor device-to-device purposes is a less likely alternative as spectrumis a scarce resource. Moreover, (dynamic) sharing between thedevice-to-device services and cellular services is more flexible andprovides higher spectrum efficiency.

Devices that want to communicate, or even just discover each other,typically need to transmit various forms of control signaling directlybetween one another. Control signaling transmitted directly betweendevices (i.e., as device-to-device communication) is referred to hereinas direct control signaling. One example of such direct controlsignaling is the so-called discovery signal (also known as a beaconsignal). A discovery signal at least carries some form of identity andis transmitted by a device that wants to be discoverable by otherdevices. Other devices can scan for the discovery signal. Once the otherdevices have detected the discovery signal, they can take theappropriate action. For example, the other devices can try to initiate aconnection setup with the device transmitting the discovery signal.

When multiple devices transmit direct control signaling (discoverysignals or any other type of direct control signaling), thetransmissions from the different devices may be time synchronized(mutually time-aligned) or unsynchronized. Synchronization could beobtained for example by receiving appropriate signals from an overlaidcellular network, or from a global navigation satellite system such as aglobal positioning system (GPS). Discovery signals transmitted by adevice within a cell for instance are typically synchronized to acell-specific reference signal transmitted by the cell. Even inunsynchronized deployments, it may be beneficial for different cells tosynchronize to each other, maintaining a time resolution up to thatobtainable from the backhaul. If the network time protocol (NTP) is thesource of synchronization, typical synchronization drifts are in theorder of +/−5 ms.

Unsynchronization could occur where discovery signals are transmittedbetween unsynchronized cells, carriers and/or public land mobilenetworks (PLMNs). According to ProSe requirements, wirelesscommunication devices belonging to one cell need to be able to discoverwireless communication devices camping on another cell. Additionally,the proximity wireless communication devices may camp on different PLMNsor different carriers. Where different cells, carriers, or PLMNs areunsynchronized, from a device-to-device communication perspective, thereare no cell boundaries.

The ProSe Study Item recommends supporting device-to-devicecommunication for out-of-network coverage wireless communicationdevices. In such case, different synchronization options are possible:wireless communication devices may synchronize to a global reference(e.g., GPS) which is in general different from the synchronizationreference of deployed networks. Alternatively, wireless communicationdevices may operate in a fully asynchronous fashion (no synchronizationreference). A further option is that clusters of wireless communicationdevices synchronize to a specific wireless communication device, such asa Cluster Head (CH). This CH provides local synchronization to itsneighbor wireless communication devices. Different clusters are notnecessarily synchronized.

Wireless communication devices may discover unsynchronized discoverysignals on a given carrier (or subband) by searching for discoverysignals in time over their configured/predefined resources. This can bedone, e.g., by time domain correlation of the received signal with thediscovery signal waveforms. This is similar to the way wirelesscommunication devices search for cells in a long-term evolution (LTE)standard for wireless communication. LTE uses a primary synchronizationsignal (PSS) and a secondary synchronization signal (SSS).

Wireless communication devices may alternate between an awake state anda sleep state (i.e., discontinuous reception (DRX)). During a sleepstate, the memory and clocks are active, but the wireless communicationdevice is configured to not monitor for any direct control signaling.During an awake state (or wake up time), the device is configured toindeed monitor for direct control signaling. Not monitoring for directcontrol signaling during the sleep state reduces the device's powerconsumption.

SUMMARY

One or more embodiments herein recognize that unsynchronized directcontrol signaling between wireless communication devices threatens thosedevices with excessive power consumption. Indeed, the unsynchronizednature of the direct control signaling suggests that a device mustmonitor for direct control signaling from another device (i.e., remainin an awake state) over a long period of time in order to ensure thatany such signaling is detected with acceptable latency. One or moreembodiments herein, however, advantageously enable the device to monitorfor unsynchronized direct control signaling without having to remain inan awake state for such a long period of time.

More particularly in this regard, embodiments herein include a firstwireless communication device in a first one of multiple groups ofwireless communication devices in a wireless communication system. Thedevices within any given group are synchronized to the same timingreference. By contrast, devices in different groups are not synchronizedto the same timing reference.

In some embodiments, for instance, the groups correspond to differentclusters of devices. Each cluster has a cluster head that is a devicecorresponding to the cluster and that assigns resources on which devicesin the cluster are to transmit direct control signaling to otherdevices. In other embodiments, though, the groups correspond todifferent cells in a cellular communication system. Radio coverage fordevices in respective cells are provided by radio network nodes.

Irrespective of whether the groups correspond to clusters or cells, thefirst device in some embodiments is configured to receive a message thatindicates, for each of one or more of the groups, a range of possiblevalues for misalignment between the timing reference of that group and acommon timing reference, where such range accounts for uncertainty inthat misalignment. Based on the one or more ranges indicated by thisreceived message, the first device determines intervals of times duringwhich direct control signaling is expected to be received at the firstdevice from one or more other devices in one or more other groups.Having made this determination, the first device advantageously adjuststhe intervals of times during which the first device is configured tooperate in an awake state to narrowly encompass the intervals of timesduring which the direct control signaling is expected to be received.The first device in this regard monitors for direct control signalingfrom other devices when operating in the awake state, but does notmonitor for such direct control signaling when operating in a sleepstate. In some embodiments, for example, the device narrowly tailors itssleep times around the timing of potential inter-group direct controlsignaling reception, so as to maximize sleep time while avoiding missinginter-group direct control signaling.

Regardless, in one embodiment, the received message indicates the rangeof possible values for misalignment between the timing reference of agiven group and the common timing reference, by indicating the maximumone of those possible values.

In one embodiment, the common timing reference is the timing referenceof a certain one of the groups.

In one embodiment, the message indicates for at least one of the groupsdifferent ranges of possible values for misalignment between the timingreference of that group and the common timing reference. These differentranges are associated with different resources configured fortransmitting control signaling directly between devices.

In other embodiments, the first device is configured to autonomouslyestimate the intervals of time during which such direct controlsignaling is expected to be received, instead of or in conjunction withreceiving the message above. In one embodiment, for example, the firstdevice is configured to receive direct control signaling from a seconddevice in a second one of the groups. Based on reception of this directcontrol signaling, the first device estimates the intervals of timeduring which direct control signaling is expected to be received at thefirst device from devices in the second group. The first device thenadjusts the intervals of time during which the first device isconfigured to operate in the awake state (for receiving direct controlsignaling from the second group) to narrowly encompass these determinedintervals of time, as described previously.

In some embodiments, this estimation entails identifying the directcontrol signaling received from the second device as having beenreceived from a device in the second group, based on extracting anidentity of the second group from the direct control signaling. Thefirst device then derives a timing reference, or range of possibletiming references, of this second group from the timing with which thedirect control signaling was received from the second device. Finally,the estimation is performed based on an assumption that devices in thesecond group transmit direct control signaling according to the derivedtiming reference, or range of possible timing references.

In one or more embodiments, the estimation comprise determining a rangeof possible values for misalignment between the timing references of thefirst and second groups, with this range accounting for uncertainty inthat misalignment. This range is determined in some embodiments forexample, based on one or more of: (i) a margin of error allowed fordevices in the first or second group to be considered as synchronized tothe same timing reference; (ii) inherent propagation delay between aradio node associated with the first or second group and devices in thatgroup; and (iii) inherent propagation delay between devices in differentgroups. At least a portion of the range of possible values formisalignment may be determined based on a communication protocolemployed for communication in and/or between the different groups.

Irrespective of how the above-mentioned intervals of time are determinedor estimated, the adjusting herein comprises in some embodimentsshortening the intervals of time during which the first device isconfigured to operate in the awake state as compared to before the firstdevice determined or estimated the intervals of times during which thedirect control signaling is expected to be received.

In one or more embodiments, the first device, as a result of theadjusting, preferentially operates in the sleep state during intervalsof time when the direct control signaling is not expected to be receivedat the first device.

In some embodiments, the first device transitions to a sleep stateearlier than nominally configured according to the adjusting. The firstdevice does so responsive to receiving and decoding direct controlsignaling during an interval of time when the first device is in anawake state.

In at least one embodiment, the first device occasionally orperiodically extends the intervals of time during which it is nominallyconfigured to operate in the awake state according to the adjusting,such that those intervals of time no longer narrowly encompass theintervals of times during which the direct control signaling is expectedto be received. But, when direct control signaling is received duringthe extended intervals of time, the first device re-adjusts theintervals of time during which it is configured to operate in the awakestate to narrowly encompass the intervals of time during which thedirect control signaling is expected to be received, accounting for thedirect control signaling received during the extended intervals of time.

In still other embodiments, the first device is nominally configuredaccording to the adjusting to operate in an awake state during aperiodically recurring time resource that narrowly encompasses theintervals of times during which it expects to receive direct controlsignaling. Responsive to determining that no direct control signalinghas been detected on the time resource for a defined amount of time, thefirst device increases the periodicity of the time resource. But,responsive to detecting that direct control signaling has been restartedon that time resource, the first device decreases the periodicity ofthat time resource.

Embodiments herein further include corresponding apparatus, computerprograms, carriers, and computer program products.

Of course, the present invention is not limited to the above featuresand advantages. Indeed, those skilled in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless communication system accordingto one or more embodiments that includes multiple groups of wirelesscommunication devices.

FIG. 2 is a block diagram of a radio access network that includesmultiple cells corresponding to the groups in FIG. 1, according to oneor more embodiments.

FIG. 3 is a logic flow diagram of a method performed by a first wirelesscommunication device according to one or more embodiments.

FIG. 4 illustrates an example of how the first wireless communicationdevice adjusts the intervals of time it is configured to operate in anawake state, according to one or more embodiments.

FIG. 5 is a logic flow diagram of a method performed by a first wirelesscommunication device according to one or more other embodiments.

FIGS. 6 and 7 illustrate examples of how the first wirelesscommunication device adjusts the intervals of time it is configured tooperate in an awake state, according to one or more other embodiments.

FIG. 8 is a block diagram of a first wireless communication deviceaccording to one or more embodiments.

DETAILED DESCRIPTION

FIG. 1 shows a wireless communication system 10 comprising multiplegroups 12 of wireless communication devices 14. More specifically, FIG.1 illustrates as an example one group 12-1 of three devices 14-1 through14-3, another group 12-2 of three devices 14-4 through 14-6, and yetanother group 12-3 of three devices 14-7 through 14-9. The devices 14within any given group 12 are synchronized to the same timing reference,at least within a predefined “acceptable” margin of error. Synchronizedin this way, the devices 14 within any given group 12 time theirtransmission and reception according to this same timing reference.Devices 14 in different groups 12, by contrast, are not synchronized tothe same timing reference, meaning that the timing references ofdifferent groups 12 (and thereby the transmission and reception timingof different groups 12) are at risk of being misaligned.

A radio node 16 is associated with each group 12. As shown, forinstance, radio node 16-1 is associated with group 12-1, radio node 16-2is associated with group 12-2, and radio node 16-3 is associated withgroup 12-3. A radio node 16 as used herein is configured to transmit andreceive radio signals, and to control one or more devices 12 within itsassociated group 12 in some capacity (e.g., by controlling the timingreference used by the group 12).

In some embodiments, for example, the different groups 12 correspond todifferent cells in a cellular communication system. In this case, theradio nodes 16 in FIG. 1 are radio network nodes (e.g., base stations)that provide radio coverage for the devices 12 in their respectivecells. This case is therefore also appropriately referred to herein as anetwork (NW) coverage case, where the devices 14 are within the coverageof a wireless communication network and communicate with the network viathe radio network nodes providing radio coverage for respective cells.

FIG. 2 broadly illustrates this network coverage case by illustratingwireless communication devices 14 being within the radio coverage ofradio network nodes 18 (e.g., base stations) in a radio access network20. The cells 22 provided by these radio network nodes 18 correspond tothe groups 12 in FIG. 1, and may correspond to different cells 22 of thesame public land mobile network (PLMN), different carrier, or differentPLMN. Devices 14 located in the same cell 22 (camping on the same cell22) are synchronized to the same timing reference and typically derivethat synchronization from the downlink in that cell 22. This ensuresthat transmissions from different devices 14 are time synchronized and,consequently, reception at a given device 14 is roughly synchronized.The timing difference is proportional to the distance and could beabsorbed by the cyclic prefix in orthogonal frequency-divisionmultiplexing (OFDM) for instance. In any event, the network coveragecase assumes that devices 14 at issue are sufficiently close to oneanother in order to communicate directly with one another, e.g., viadevice-to-device communication, irrespective of whether the devices 14are served by different radio network nodes 18. Such assumption oftenproves true, for instance, in heterogeneous networks that employ macroradio network nodes as well as pico, micro, etc. radio network nodes.

In other embodiments, the different groups 12 in FIG. 1 correspond todifferent clusters of devices 14 that are synchronized to the sametiming reference. This may be the case for instance where devices 14 areeither not configured to communicate with any wireless communicationnetwork (or are simply not within the coverage of such a network), butthe devices 14 within any given cluster are nonetheless withincommunication range of each other for device-to-device communication. Inthis case, which is also appropriately referred to herein as the lack ofnetwork coverage case, the radio nodes 16 in FIG. 1 are so-calledcluster heads. A cluster head as used herein is a wireless communicationdevice 14 that not only belongs to a group 12 of devices 14, but alsocontrols the devices 14 in that group 12 in some capacity; that is, thecluster head acts as the head of a cluster of devices 14 that aresynchronized to the same timing reference. The cluster head in someembodiments for instance has special control authority to assign the“direct control” resources on which devices 14 in the cluster are totransmit direct control signaling to other devices 14. That is, ratherthan the devices 14 themselves autonomously deciding on which resourcesto transmit direct control signaling (e.g., possibly within a subset ofpreconfigured resources, such as a certain subband), the cluster headmakes such decision on behalf of the cluster as a whole. The clusterhead alternatively or additionally controls the devices 14 in a group 12by controlling the timing reference used by that group 12.

Irrespective of whether the groups 12 in FIG. 1 constitute cells orclusters, devices 14 transmit so-called direct control signaling withother devices 14 in the same or a different group 12. Direct controlsignaling in this regard refers to control signaling transmitteddirectly between devices 14, i.e., as device-to-device (D2D)communication that does not involve any intermediate nodes. One exampleof such direct control signaling is a so-called discovery signal (alsoknown as a beacon signal) that a device 14 transmits in order to bediscovered by other devices 14 in proximity. Any embodiments hereinfocusing on such discovery signals are equally applicable to other typesof direct control signaling unless indicated otherwise. In at least someembodiments, a radio node 16 for a group 12 configures resources fortransmission of direct control signaling, such as discovery signals,according to a periodic, regular, sparse in time, or otherwisepredefined pattern. The time (and possibly frequency) resources fordirect control signaling transmission/reception in any given group 12are defined with respect to the timing reference of that group 12. Withdevices 14 in any given group timing their transmission and receptionaccording to the same timing reference, direct control signaling betweenthose devices 14 is synchronized in nature. By contrast, since devices14 in different groups 12 time their transmission and receptionaccording to different timing references, direct control signalingbetween devices 14 in different groups 12 is unsynchronized in nature.

In the interest of power efficiency, any given wireless communicationdevice 14 operates in either an awake state or a sleep state accordingto an awake-sleep state cycle (e.g., DRX cycle). In the awake state, adevice 14 monitors for direct control signaling from other devices 14,such as by turning on one or more receivers. In a sleep state, bycontrast, a device 14 does not monitor for such direct controlsignaling, such as by turning off one or more receivers.Correspondingly, a device 14 conserves more power when operating in asleep state than when operating in an awake state. The unsynchronizednature of inter-group direct control signaling, however, threatens adevice's ability to conserve power in this way.

A device 14 according to one or more embodiments herein however monitorsfor inter-group direct control signaling in a power-efficient mannerdespite the unsynchronized nature of such signaling. In this regard, anygiven device 14 nominally transmits and monitors for direct controlsignaling according to the timing reference of its group 12. This wouldotherwise suggest that the device 14 must monitor for inter-group directcontrol signaling over a long period of time (since the device 14otherwise has no information about when to expect that inter-groupcontrol signaling due to its unsynchronized nature). One or moreembodiments herein advantageously enable the device 14 to monitor forunsynchronized direct control signaling without having to remain in anawake state for such a long period of time.

FIG. 3 for example illustrates embodiments that enable a device 14 to dothis by providing the device 14 with a particular kind of message.Specifically in this regard, a first wireless communication device 14-1in group 12-1 is configured to implement the method 100 shown in FIG. 3,according to one or more embodiments. The first device 14-1 isconfigured to receive a particular message (Block 110), e.g., from theradio node 16-1 (i.e., base station or cluster head) associated with thedevice's group 12-1. This particular message indicates, for each of oneor more of the groups 12, a range of possible values for misalignmentbetween the timing reference of that group 12 and a common timingreference, where such range accounts for uncertainty in thatmisalignment. In some embodiments, the common timing reference is thetiming reference of a certain group 12, meaning that the messagedirectly indicates the range of possible misalignment between differentgroups' timing references. In other embodiments, the common timingreference is an absolute timing reference (e.g., a global or universaltiming reference divorced from any group 12). In this case, the messagestill indicates the range of possible misalignment between differentgroups' timing references, but the message indicates this onlyindirectly via the absolute timing reference or in conjunction withother information. In either case, therefore, the message providesinformation effective for the first device 14-1 to determine a range ofpossible timing misalignment between different groups 12 (e.g., betweengroup 12-1 and group 12-2 and/or between group 12-1 and group 12-3),accounting for uncertainty in that misalignment. Where the groups 12correspond to cells 22, for instance, the message in at least someembodiments includes an indication of the synchronizationerror/inaccuracy between the serving cell and other cells in proximity,whose devices 14 are transmitting direct control signaling of interest.

Regardless, the first device 14-1 is further configured to determine,based on the one or more ranges indicated by the received message,intervals of times during which direct control signaling is expected (orlikely) to be received from one or more devices 12 in one or more othergroups 12 (i.e., group 12-2 and/or group 12-3) (Block 120). That is, thefirst device 14-1 determines from the indicated range(s) that, if adevice (e.g., device 14-4) in a different group (e.g., group 12-2)transmits such direct control signaling, that signaling should bereceived at the first device 14-1 within the determined intervals oftime, given the range of possible synchronization misalignment betweenthe groups at issue (e.g., groups 12-1 and 12-2). In any event, havingmade this determination, the first device 14-1 adjusts the intervals oftimes during which it is configured to operate in an awake state tonarrowly encompass the intervals of times during which the directcontrol signaling is expected (or likely) to be received (Block 130). Insome embodiments, for example, the first device 14-1 preferentiallyoperates in the sleep state during intervals of time when, according tothe above determination, no inter-group direct control signaling isexpected or likely to be received. This effectively conserves devicepower while still ensuring acceptable latency in detecting inter-groupdirect control signaling.

In at least a general sense, the message that the first device 14-1receives in FIG. 3 effectively describes the unsynchronized nature ofinter-group direct control signaling in the system 10, accounting foruncertainty in that unsynchronized nature. The information in this senseeffectively makes the device 14-1 aware of the set of resources in timewhere direct control signaling (e.g., discovery signals) are expected,even from devices in other (e.g., neighboring) groups 12. The firstdevice 14-1 according to FIG. 3 advantageously exploits this informationabout the signaling' s unsynchronized nature in order to more narrowlytailor the timing of its direct control signaling monitoring (e.g.,awake-sleep state cycle or DRX cycle) to the timing of potential directcontrol signaling reception.

In some embodiments, the received message indicates a range of possiblevalues for misalignment between the timing reference of a given group 12and a common timing reference, by indicating the maximum one of thosepossible values. That is, the range is indicated in terms of the maximumpossible timing misalignment (e.g., with a minimum possible timingmisalignment being known or predefined). Where the groups 12 correspondto cells 22, for instance, the timing range may describe the maximumtiming misalignment between cells 22 in proximity.

No matter the particular implementation, though, the message notablyindicates the timing misalignment range, as opposed to a single timingalignment offset, in order to account for one or more sources ofuncertainty in the misalignment. In some embodiments, one such source ofuncertainty originates from the margin of error allowed for devices 14in the same group 12 to be considered as synchronized to the same timingreference. Indeed, this margin of error effectively allows a range ofpossible values for misalignment between the actual timing referencesused by devices 14 in the same group 12. The timing misalignment rangeindicated by the message encompasses and otherwise accounts for thismargin of error.

Alternatively or additionally, another source of uncertainty originatesfrom inherent propagation delay between a group's radio node 16 (e.g.,base station or cluster head) and the devices 14 in that group 12.Indeed, this propagation delay affects a device's perception of thegroup's timing reference to an unknown extent.

As yet another example, another source of uncertainty originates frominherent propagation delay between devices 14 in different groups 12.This unknown propagation delay affects a device's perception of theextent of misalignment between the groups' timing references.

The sources of uncertainty accounted for by the message are of coursenot limited to the above examples. That is, in general, one or moreembodiments herein envision that the one or more ranges of misalignmentindicated by the message account for any or all sources of suchuncertainty, including those not explicitly outlined by the aboveexamples.

With an understanding of this uncertainty, the first device 14-1according to some embodiments advantageously shortens the intervals oftime during which it is configured to operate in the awake state, e.g.,as compared to before the first device 14-1 determined the intervals oftime during which direct control signaling is expected to be received.Consider for example the awake-sleep state (i.e., DRX) cycle shown inFIG. 4. As shown, the awake state timing of the cycle has been adjustedby the first device 14-1 from a more power-inefficient timing 20 inwhich the device 14-1 operates in the awake state for long periods oftime to a more power-efficient timing 22 in which the device 14-1operates in the awake state for shorter periods of time. Indeed, ratherthan having its awake state time intervals (referred to here as the “DRXWindows”) broadly encompass times during which direct control signalingwould have to be received under the most conservative of possibilities,the first device 14-1 adjusts its DRX window size to narrowly encompassintervals of times during which direct control signaling is expected (orlikely) to be received (e.g., from device 14-2 in group 12-1 and device14-4 in group 12-2). That is, the device 14-1 adjusts its DRX periodsuch that it is awake during the DC windows and asleep during theremaining time, for the purpose of direct control signaling reception.Of course, the device 14-1 may still wake up during other periods forperforming different operations than direct control signaling reception,but by doing so the device 14-1 is awake for receiving direct controlsignaling only for the intervals of time when such direct controlsignaling is expected to be received. These intervals of time, asdescribed above, notably account for uncertainty in the timingmisalignment between groups 12-1 and 12-2. Where the groups 12 in FIG. 1correspond to cells 22, for instance, the width of the DC windows maydepend on the synchronization margin and/or synchronization accuracybetween cells 22, and can be on the order of +/− some milliseconds. Inany event, this means that there will still be some inherent powerinefficiency in the first device's inter-group signaling monitoring(i.e., the DRX window may still be longer than it would have been hadthere been absolute certainty in the timing misalignment). But the aboveawake state timing adjustment reduces this power inefficiency in someembodiments to the extent possible given this uncertainty.

In the interest of reducing this uncertainty and thereby increasingpower efficiency, a wireless communication device 14 according to one ormore other embodiments herein autonomously estimates the intervals oftime during which inter-group direct control signaling is expected (orlikely) to be received, instead of or in conjunction with receiving themessage above. A first wireless communication device 14-1 according tosome embodiments therefore is a “smart” device that alternatively oradditionally implements the method 200 shown in FIG. 5.

As shown in FIG. 5, the first device 14-1 receives direct controlsignaling from a second device 14-4 in a second group 12-2 (Block 210).The first device estimates, based on the timing with which the directcontrol signaling was received from the second device 14-4, intervals oftimes during which the first device 14-1 expects to receive directcontrol signaling from devices 14 in the second group 12-2 (Block 220).Similar to embodiments above, the first device 14-1 then adjusts theintervals of times during which it is configured to operate in an awakestate (for receiving signaling from the second group 12-2) to narrowlyencompass the intervals of times during which the direct controlsignaling is expected to be received from the second group 12-2 (Block230).

In general, therefore, the first device 14-1 estimates the timing ofdirect control signaling reception for a given group 12 based on when itsuccessfully detected direct control signaling from one or more devices14 within that group 12. In one embodiment, for example, the firstdevice 14-1 successfully detects at least some direct control signalingfrom devices (e.g., device 14-4) belonging to a given group 12. Thefirst device 14-1 then estimates the timing associated with that givengroup 12 and shortens its receiver window around the expected receptiontiming instances (e.g., in a way similar to that shown in FIG. 4). Byautonomously estimating the correct timing for different groups (e.g.,cells or clusters) based on actual direct control signaling reception,the reception windows in FIG. 5 may be narrower than the ones that theones attained in FIG. 3.

As shown in FIG. 5, the first device 14-1 in some embodiments exploitsgroup identity information conveyed by direct control signaling in orderto accomplish the above described estimation. Specifically, the firstdevice 14-1 in these embodiments identifies direct control signaling ashaving been received from the second device 14-4 in a particular group,i.e., Group 12-2, based on extracting an identity of that group (e.g., acell identity, PLMN identity, and/or cluster identity) from the directcontrol signaling (Block 212). This approach of course relies on devices14 to transmit information about their group identity in order to assistother devices 14 in associating their group with the correct timing. Inany event, having accomplished this identification, the first device14-1 then derives a timing reference (or range of possible timingreferences) of the particular group 12-2 from the timing with which thedirect control signaling was received (Block 214). Based on theassumption that devices 14 in the particular group 12-2 transmit directcontrol signaling according to the derived timing reference, the firstdevice 14-1 advantageously estimates the intervals of time during whichdirect control signaling is expected to be received at the first device14-1 from the devices 14 in the particular group 12-2.

In at least some embodiments, the above estimation delays operation in apower-efficient awake-sleep state until sufficient direct controlsignaling is received for estimating the timing reference at issue. Inthis sense, then, the autonomous estimation approach proves lesspower-efficient than the signaling approaches described with respect toFIG. 3. However, the above estimation in some embodiments provides morecertainty in the timing misalignment between groups 12, meaning that thewireless device 14-1 more narrowly tailors its awake-sleep state timingto the potential direct control signaling reception as compared to thatperformed in the signaling approaches. Indeed, in at least someembodiments, the first device 14-1 performs its estimation in Block 220of FIG. 5 by itself determining a range of possible values formisalignment between the timing references of the first and secondgroups 12-1, 12-2, with the range accounting for uncertainty in thatmisalignment as described above. That is, the first device 14-1 itselfeffectively estimates or otherwise characterizes the uncertainty bytaking into account one or more of the sources of uncertainty mentionedabove. For example, in one embodiment, the first device 14-1 itselfdetermines the range of possible values for the misalignment based onone or more of (i) a margin of error allowed for devices 14 in the firstor second group 12-1, 12-2 to be considered as synchronized to the sametiming reference; (ii) inherent propagation delay between a radio node16 associated with the first or second group 12-1, 12-2 and the devices14 in that group; and (iii) inherent propagation delay between devices14 in different groups 12. The first device 14-1 may for instance insome embodiments determine a portion of the range of misalignmentattributable to any given source of uncertainty based on one or moreparameters associated with communication in and/or between the groups 12at issue (e.g., the communication protocol employed).

Irrespective of whether the autonomous estimation approach of FIG. 5 orthe signaling approach of FIG. 3 is employed, the first device 14-1 insome embodiments implements certain other features in order to furtherconserve power and/or to ensure direct control signaling reception. Inone or more embodiments, for example, the first device 14-1 furtherconserves power by transitioning to a sleep state earlier than nominallyconfigured to according to the adjusted awake-sleep state cycle,responsive to direct control signaling detection/decoding. Specifically,the first device 14-1 in some embodiments receives and decodes directcontrol signaling during an interval of time when the first device 14-1is in an awake state. Despite the first device 14-1 still beingnominally configured to be in the awake state (i.e., according to thenarrowly tailored awake-sleep state cycle), the first device 14-1 makesan early transition to the sleep state upon such direct controlsignaling decoding, based on an assumption or knowledge that no furtherdirect control signaling is expected to be received. For example, oncethe first device 14-1 has correctly decoded any direct control signalingwithin the DC window, it turns into sleeping mode even though the DCwindow is not expired yet. This is because the corresponding directcontrol signaling has already been received and there is no need for thefirst device 14-1 to keep awake and consume battery for that DC windowinstance (e.g., corresponding to a certain discovery instance where thedirect control signaling is a discovery signal). Regardless, it least insome embodiments, this early transition to the sleep state does nototherwise affect the nominally configured awake-sleep state cycle of thefirst device 14-1.

According to one or more other embodiments, the first device 14-1occasionally or periodically extends the intervals of times during whichit is nominally configured to operate in an awake state, so that thoseintervals of time are no longer narrowly tailored as described above.The first device 14-1 does this in the interest of subsequently updatingits narrow tailoring of awake time intervals. Indeed, when directcontrol signaling is received during those extended intervals of time,the first device 14-1 re-adjusts the intervals of time during which itis configured to operate in an awake state to narrowly encompass theintervals of time during which the direct control signaling is expectedto be received, accounting for the direct control signaling receivedduring those extended intervals of time. This way, the first device 14-1does not miss direct control signaling from new groups 12 of devices 14that have come within communication range since the first device's lastnarrow tailoring of its awake time intervals (which did not account forsuch new groups 12).

Indeed, once the reception window has been shortened according to, e.g.,one of the embodiments above, the first device 14-1 risks missing newdirect control signaling that falls outside the narrow reception window.This can be the case, e.g., when the first device 14-1 moves inproximity of new cells or clusters. In order to avoid such problem, asjust described, the first device 14-1 periodically or occasionallyperforms a longer search for direct control signaling based on a longerwake up time. During such wake up time the first device 14-1 is likelyto be able to acquire timing for all or at least most devices 14 inproximity that are transmitting direct control signaling. The longerwake up time in some embodiments that involve discovery signaling forinstance corresponds approximately to a full discovery cycle where alldevices 14 involved in discovery transmit their discovery signals atleast once. Once the timing for such devices 14 is acquired, the DRXwindow in some embodiments is shortened accordingly.

Consider the example in FIG. 6. As shown in Block 1, the first device14-1 initially tailored its awake time intervals to the expected timingof direct control signaling received from device 14-4 in group 12-2.Since doing so, however, the first device 14-1 has come withincommunication range of group 12-3, of which device 14-7 is assumed hereto be a member. In order to ensure that the first device 14-1 detectsdirect control signaling in such a situation, the first device 14-1 isconfigured to occasionally or periodically perform a longer awake (DRX)cycle, as shown in Block 2. During such a longer awake cycle, the firstdevice 14-1 detects direct control signaling from device 14-7 with adifferent timing (since device 14-7 belongs to a different group thandevice 14-4). Thereafter, of course, the first device 14-1 narrowlytailors its awake (DRX) cycles around the expected direct controlsignaling reception timing of both Groups 12-2 and 12-3 (i.e., fordevices 14-4 and 14-7). As shown in Block 3, for example, the firstdevice 14-1 in some embodiments is configured to even operate in a sleepstate between the expected reception windows for groups 12-2 and 12-3(e.g., in an aperiodic fashion).

Still one or more other embodiments under certain circumstances drop atleast some of the time intervals during which the first device 14-1 isnominally configured to operate in an awake state. These embodimentsprove advantageous for instance when, due to mobility and other reasons,devices 14 may cease to be in proximity or may stop transmitting directcontrol signaling. In this case, it would be a waste of energy for thereceiving device 14 to keep monitoring resources where no device 14 istransmitting anymore.

According to these embodiments, therefore, the first device 14 isnominally configured to operate in an awake state during a periodicallyrecurring time resource that narrowly encompasses the intervals of timesduring which the first device 14-1 expects to receive direct controlsignaling. Responsive to determining that no direct control signalinghas been detected on that time resource for a defined amount of time,the first device 14-1 increases the periodicity of the time resource.That is, the interval between the periodic occurrences of that timeresource (i.e., the interval between the periodic occurrences of the DRXwindow instances) is increased, rather than completely droppingmonitoring of that resource, just in case direct control signaling onthat resource is restarted. Indeed, responsive to detecting that directcontrol signaling has been restarted on the time resource, the firstdevice 14-1 decreases the periodicity of the time resource again.

FIG. 7 illustrates these embodiments with a simple example. As shown,the first device 14-1 is nominally configured to operate in an awakestate during a periodically recurring time resource that narrowlyencompasses the intervals of times during which the first device 14-1expects to receive control signaling from devices in group 12-2,including as shown device 14-4. At time t1, the first device 14-1determines that no direct control signaling has been detected on thistime resource. Responsive to this, the first device 14-1 increases theperiodicity of the time resource (i.e., so that it has a longerperiodicity as shown). Eventually, at time t2, another device 14-5 ingroup 12-2 begins transmitting direct control signaling. The firstdevice 14-1 misses the first direct control signaling transmission attime t2 because the first device 14-1 is conserving power with thelonger periodicity. However, at time t3, the first device 14-1 detectsdevice 14-5's direct control signaling transmission and resumes itsshorter periodicity as before.

One particular implementation of such embodiments involves the firstdevice 14-1 reseting a timer whenever direct control signaling isdetected corresponding to a certain timing instance and DRX window. Ifthe timer exceeds a certain value before new direct control signaling isdetected within the same DRX window (i.e., no DC signaling has beendetected within the DRX window for a certain time), the interval betweenDRX windows is increased. This is to save device power and to ensurethat it is possible to detect if activity is restarted on that resourceafter some time. If activity restarts, the monitoring interval, i.e. theinterval between DRX windows is decreased again in some embodiments.

Note that, in one or more embodiments, there may be different ranges ofpossible values for misalignment between the timing reference of a groupand the common timing reference. These different ranges may for instancebe associated with different resources configured for transmittingdirect control signaling between devices.

In view of the above modifications and variations, FIG. 8 shows anexample embodiment of a wireless communication device 14. The wirelesscommunication device 14 comprises one or more processing circuits 30configured to perform the method in FIG. 3 and/or FIG. 5. The wirelesscommunication device 14 also includes one or more radio transceivercircuits 32 configured to both transmit and receive radio signals. Theone or more radio transceiver circuits 32, for example, includes variousradio-frequency components (not shown) to receive and process radiosignals from other radio nodes, via one or more antennas, using knownsignal processing techniques. Notably, the one or more radio transceivercircuits 32 are also configured to directly transmit and receive radiosignals to/from other wireless communication devices 14, e.g., viadevice-to-device communication.

The wireless communication device 14 in some embodiments furthercomprises one or more memories 34 for storing software to be executedby, for example, the one or more processing circuits 30. The softwarecomprise instructions to enable the one or more processing circuits 30to perform the method as shown in FIG. 3 and/or FIG. 5. The memory 34may be a hard disk, a magnetic storage medium, a portable computerdiskette or disc, flash memory, random access memory (RAM) or the like.Furthermore, the memory 34 may be an internal register memory of aprocessor.

Of course, not all of the steps of the techniques described herein arenecessarily performed in a single microprocessor or even in a singlemodule. Thus, a more generalized control circuit configured to carry outthe operations described above may have a physical configurationcorresponding directly to the processing circuit(s) 30 or may beembodied in two or more modules or units. The wireless communicationdevice 14 may for instance include different functional units, eachconfigured to carry out a particular step of FIGS. 3 and/or FIG. 5.

Those skilled in the art will also appreciate that embodiments hereinfurther include a corresponding computer program. The computer programcomprises instructions which, when executed on at least one processor ofa wireless communication device 14, cause the device 14 to carry out anyof the processing described above. Embodiments further include a carriercontaining such a computer program. This carrier may comprise one of anelectronic signal, optical signal, radio signal, or computer readablestorage medium. Those skilled in the art will appreciate that such acomputer program according to some embodiments comprises one or morecode modules contained in memory 34, each module configured to carry outa particular step of FIGS. 3 and/or FIG. 5.

As used herein, the term “wireless communication device” 14 is anydevice configured to communicate wirelessly with another node and tocommunicate directly with another such wireless communication device 14(i.e., via device-to-device communication). A wireless communicationdevice 14 therefore includes a user equipment (UE), a mobile phone, acellular phone, a Personal Digital Assistant (PDA) equipped with radiocommunication capabilities, a smartphone, a laptop or personal computer(PC) equipped with an internal or external mobile broadband modem, atablet PC with radio communication capabilities, a portable electronicradio communication device, a sensor device equipped with radiocommunication capabilities, a machine-to-machine device, or the like.

Also as used herein, the term “radio network node” refers to a radionode that is part of a radio access network 20. A radio network node,for example, includes an eNB in LTE, a control node controlling one ormore remote radio units (RRUs), a radio base station 16, an accesspoint, or the like. The radio network node in some embodiments isconfigured to operate over a so-called system bandwidth. A portion ofthis system bandwidth in some embodiments in reserved, statically ordynamically, for D2D communication. Hence, a DC bandwidth is availablefor assignment to for example DC messages.

Further, as used herein, a timing reference includes any reference inthe time domain that functions as a common source for time-domainsynchronization. A timing reference may include for instance the timingof a defined transmission or reception window. In LTE, for example, suchincludes the timing of an LTE subframe in some embodiments.

Still further, different direct control signaling resources herein insome embodiments have the same timing window widths. In otherembodiments, though, different direct control signaling resources areassociated with different timing windows widths.

Those skilled in the art will also appreciate that the various“circuits” described may refer to a combination of analog and digitalcircuits, including one or more processors configured with softwarestored in memory and/or firmware stored in memory that, when executed bythe one or more processors, perform as described above. One or more ofthese processors, as well as the other digital hardware, may be includedin a single application-specific integrated circuit (ASIC), or severalprocessors and various digital hardware may be distributed among severalseparate components, whether individually packaged or assembled into asystem-on-a-chip (SoC).

Thus, those skilled in the art will recognize that the present inventionmay be carried out in other ways than those specifically set forthherein without departing from essential characteristics of theinvention. Moreover, the above embodiments are able to be implementedindependently or in combination with one another. The presentembodiments are thus to be considered in all respects as illustrativeand not restrictive.

What is claimed is:
 1. A method implemented by a first wirelesscommunication device in a first one of multiple groups of wirelesscommunication devices in a wireless communication system, where devicesin any given group are considered as synchronized to the same timingreference and devices in different groups are not considered as besynchronized to the same timing reference, the method comprising:receiving synchronization information that indicates, for each of one ormore of the groups, a range of possible values for misalignment betweenthe timing reference of devices in that group and a serving cell, therange accounting for uncertainty in that misalignment; and monitoringfor reception of control signaling from one or more devices in one ormore other groups, based on intervals of times during which the controlsignaling is expected to be received at the first device, wherein theintervals of times during which the control signaling is expected to bereceived at the first device are based on the one or more rangesindicated by the synchronization information.
 2. The method of claim 1,wherein said monitoring comprises monitoring for reception of controlsignaling from a device in a second group, for discovery of a link tothat device for device-to-device communication, based on an interval oftime during which that control signaling is expected to be received atthe first device.
 3. The method of claim 1, wherein the synchronizationinformation indicates the range of possible values for misalignmentbetween the timing reference of devices in a group and a serving cell byindicating a window of time for devices in that group, wherein the firstdevice is configured to assume that control signaling from a device inthe group will be received within the indicated window with respect to aset of control signaling resources defined in time for the group.
 4. Themethod of claim 1, wherein the wireless communication system comprises acellular communication system, the system including radio network nodesthat provide radio coverage for devices in respective cells, wherein thegroups correspond to different cells in the system.
 5. The method ofclaim 1, wherein the range of possible values for misalignment betweenthe timing reference of devices in a group and the serving cell accountsfor one or more of: a margin of error allowed for devices in the groupto be considered as synchronized to the same timing reference; inherentpropagation delay between a radio node associated with the group and thedevices in that group; and inherent propagation delay between devices indifferent groups.
 6. The method of claim 1, wherein said monitoringcomprises switching a receiver on at a time that depends on theintervals of times.
 7. A first wireless communication device configuredto belong in a first one of multiple groups of wireless communicationdevices in a wireless communication system, where devices in any givengroup are considered as synchronized to the same timing reference anddevices in different groups are not considered as be synchronized to thesame timing reference, the first wireless communication devicecomprising: processing circuitry configured to: receive synchronizationinformation that indicates, for each of one or more of the groups, arange of possible values for misalignment between the timing referenceof devices in that group and a serving cell, the range accounting foruncertainty in that misalignment; and monitor for reception of controlsignaling from one or more devices in one or more other groups, based onintervals of times during which the control signaling is expected to bereceived at the first device, wherein the intervals of times duringwhich the control signaling is expected to be received at the firstdevice are based on the one or more ranges indicated by thesynchronization information.
 8. The first wireless communication deviceof claim 7, wherein the processing circuitry is configured to monitorfor reception of direct control signaling from a device in a secondgroup, for discovery of a link to that device for device-to-devicecommunication, based on the determined interval of time during whichthat direct control signaling is expected to be received at the firstdevice.
 9. The first wireless communication device of claim 7, whereinthe synchronization information indicates the range of possible valuesfor misalignment between the timing reference of a group and a commontiming reference by indicating a window of time for that group, whereinthe first device is configured to assume that direct control signalingfrom a device in the group will be received within the indicated windowwith respect to a set of direct control signaling resources defined intime for the group.
 10. The first wireless communication device of claim7, wherein the wireless communication system comprises a cellularcommunication system, the system including radio network nodes thatprovide radio coverage for devices in respective cells, wherein thegroups correspond to different cells in the system.
 11. The firstwireless communication device of claim 7, wherein the range of possiblevalues for misalignment between the timing reference of a group and thecommon timing reference accounts for one or more of: a margin of errorallowed for devices in the group to be considered as synchronized to thesame timing reference; inherent propagation delay between a radio nodeassociated with the group and the devices in that group; and inherentpropagation delay between devices in different groups.
 12. The firstwireless communication device of claim 7, wherein the processingcircuitry is configured to switch a receiver on at a time that dependson the determined intervals of times.
 13. A non-transitory computerreadable medium on which is stored software instructions for controllinga first wireless communication device in a first one of multiple groupsof wireless communication devices in a wireless communication system,where devices in any given group are considered as synchronized to thesame timing reference and devices in different groups are not consideredas be synchronized to the same timing reference, the softwareinstructions which, when run on one or more processors of the firstwireless communication device, cause the first wireless communicationdevice to: receive synchronization information that indicates, for eachof one or more of the groups, a range of possible values formisalignment between the timing reference of devices in that group and aserving cell, the range accounting for uncertainty in that misalignment.14. The non-transitory computer readable medium of claim 13, wherein thesoftware instructions, when run on the one or more processors of thefirst wireless communication device, further cause the first wirelesscommunication device to: monitor for reception of control signaling fromone or more devices in one or more other groups, based on intervals oftimes during which the control signaling is expected to be received atthe first device, wherein the intervals of times during which thecontrol signaling is expected to be received at the first device arebased on the one or more ranges indicated by the synchronizationinformation.
 15. The non-transitory computer readable medium of claim13, wherein the software instructions, when run on the one or moreprocessors of the first wireless communication device, cause the firstwireless communication device to monitor for reception of direct controlsignaling from a device in a second group, for discovery of a link tothat device for device-to-device communication, based on the determinedinterval of time during which that direct control signaling is expectedto be received at the first device.
 16. The non-transitory computerreadable medium of claim 13, wherein the synchronization informationindicates the range of possible values for misalignment between thetiming reference of a group and a common timing reference by indicatinga window of time for that group, wherein the first device is configuredto assume that direct control signaling from a device in the group willbe received within the indicated window with respect to a set of directcontrol signaling resources defined in time for the group.
 17. Thenon-transitory computer readable medium of claim 13, wherein thewireless communication system comprises a cellular communication system,the system including radio network nodes that provide radio coverage fordevices in respective cells, wherein the groups correspond to differentcells in the system.
 18. The non-transitory computer readable medium ofclaim 13, wherein the range of possible values for misalignment betweenthe timing reference of a group and the common timing reference accountsfor one or more of: a margin of error allowed for devices in the groupto be considered as synchronized to the same timing reference; inherentpropagation delay between a radio node associated with the group and thedevices in that group; and inherent propagation delay between devices indifferent groups.
 19. The non-transitory computer readable medium ofclaim 13, wherein the software instructions, when run on the one or moreprocessors of the first wireless communication device, cause the firstwireless communication device to switch a receiver on at a time thatdepends on the determined intervals of times.